This invention relates generally to the valve art and, more particularly, to valve assemblies designed to minimize or eliminate cavitation and vapor formation caused by a pressure drop across the valve assembly.
The phenomenon of cavitation in valves has been a constraint in their application for many years. In some cases, valves are called upon to withstand large pressure head drops under high-velocity, high-flow conditions. However, in such cases, if local pressures within the valve drop below the vapor pressure of water, cavitation can result and cause serious performance problems.
In most cases, valves provide a physical constriction in the flow of fluid through a hydraulic system. In general, the velocity of the fluid increases as it passes through the constriction because the flow area (i.e., the cross-sectional area) at the constriction is smaller than the flow area just upstream and just downstream of the constriction. This increase in velocity is accompanied by a decrease in pressure. If the differences in velocity are considerable, the pressure drop can also be considerable. In high-velocity high-flow situations, low-pressure regions tend to occur just downstream of the valve. If flow rates are high enough, these regions are likely to initiate cavitation.
The mechanism for cavitation entails the formation of small vapor nuclei, their subsequent growth within low-pressure regions of a flow, and their violent collapse as they eventually enter regions of higher pressure. The energy released by the collapse of the vapor cavities typically causes noise and vibration within a hydraulic system. Strong cavitation near physical boundaries (e.g., valve or pipe walls) can, over a period of time, cause serious damage or even failure of system components. Regions of low pressure in valves typically occur immediately downstream from an abrupt change in a valve's interior geometry. The flow in these regions tends to be highly turbulent with numerous eddies.
Attempts have been made to minimize or eliminate the occurrence of cavitation caused by significant pressure drops across valves. For example, decades ago, experiments were conducted wherein water recirculation manifolds were built into the downstream side of partially open conical-plug valves in an effort to minimize cavitation. The premise was that the low-pressure high-velocity flow downstream of the throttling cone valve would "suck" water through the passages of the manifold an into the outlet side of the valve body, thereby raising the pressure of the water at the downstream side of the valve so as to minimize or eliminate cavitation. However, testing showed that these water recirculation manifolds provided no significant benefit. Metropolitan Water District of Southern California, Report No.854 (October 1968) and Report No.855 (June 1969).
One problem with these water recirculation manifolds is that they were installed on the downstream side of the partially open valve in the area of stagnant flow, 180 degrees opposite the area of the high-velocity flow discharging from the partially open valve (the lower part of the pipe shown in the accompanying figures). Consequently, the manifolds provided no significant benefit because they were not located in the area where cavitation is most likely to occur (i.e., the area of high-velocity low-pressure flow). The manifolds were not installed in the area of high-velocity flow (the area adjacent the upper part of the pipe shown in the accompanying figures) because doing so would have produced reverse flow and ram pressure in the manifold due to the angled impact of the throttled valve flow against the holes of the manifold. The holes of these manifolds were flush with the interior surface of the valve body. Consequently, they would have produced negligible benefits, regardless of whether their location was in the area of high-velocity flow discharging from the partially open valve or in the area of stagnant flow.
Other attempts to minimize the occurrence of cavitation involved recirculation manifolds connected to atmospheric air. The premise, as with the water recirculation manifolds, was that the low-pressure high-velocity flow downstream of the throttling cone valve would "suck" atmospheric air through the passages of the manifold and into the outlet side of the valve body, thereby raising the pressure of the water at the downstream side of the valve so as to minimize or eliminate cavitation. However, similar problems were encountered and testing showed no significant benefits. Again, the manifolds were installed on the downstream side of the partially open valve in the area of stagnant flow, 180 degrees opposite the area of the high-velocity flow discharging from the partially open valve. Consequently, they provided no significant benefit because they were not located in the area of high-velocity low-pressure flow. The air manifolds were not installed adjacent the area of high-velocity flow, again because of the problem of reverse flow and ram pressure due to the angled impact of the throttled valve flow against the holes of the manifold, which were flush with the interior of the valve body.